The 30-year-old American's win, with Asafa Powell second in 9.88 seconds, will keep the pressure on Jamaican Usain Bolt, who competes in the 200 metres at the Paris Diamond League meet on Saturday.
Bolt bowed out of the Athletissima event.
Gay has long raced in the shadow of Bolt, a two-time Olympic treble gold medal winner, but the American has now set the three fastest times of the season ahead of the August 10th to 18th World Athletics Championships.
"I wanted to finish strong and stay healthy," said Gay, who also set a world-leading 19.74 seconds when winning the 200 metres at the US national championships.
"I'm feeling good and I think I have a good chance against anybody (in the worlds)."
Ukrainian athletes had a night to remember, Mariya Ryemyen surprising the field in the women's 200 metres by winning in 22.61 seconds, ahead of fancied American Carmelita Jeter.
But it was in the field where there were fireworks, as a victorious Bohdan Bondarenko leapt 2.41 metres, the best high jump in the world since 1994.
The 23-year-old Bondarenko even had a shot at a world record height of 2.46 metres, one centimetre higher than Cuban Javier Sotomayor's best set back in 1993.
"Of course I'm very happy with my jumps tonight," said Bondarenko.
"It's tough to take in what I've done," he said..
"I can't explain my progression, it's simply great for me."
But there was a rare off night for France's Olympic pole vault champion Renaud Lavillenie, who failed his three attempts at an opening height of 5.62 metres.
"Wind is not an excuse," the two-time world bronze medallist said.
"It's frustrating. What's annoying is that at no moment did I find my recent rhythm and drive."
Ethiopian tyro Mohammed Aman sent out a warning to Kenyan rival David Rudisha with an impressive victory in the men's 800 metres in a season's best of 1:43.33.
The 19-year-old produced his trademark final strait kick to go past Frenchman Pierre-Ambroise Bosse in style.
American Dawn Harper-Nelson claimed victory in the women's 100-metre hurdles in 12.53 seconds in a US 1-2-3-4, with Australia's London 2012 gold medallist Sally Pearson back in seventh in 12.69 seconds.